In a conventional plasma process, plasma is activated by reactive gaseous species so that a desired film can be produced on the substrate or removed from the substrate. The chemically reactive species includes ions, neutral atoms, and molecules. When a chemical process, such as plasma etching, is involved, reactive species interact chemically or physically with the material to be etched.
The chemically reactive species are chosen carefully so that the species are specific to the surface of the substrate that it is going to interact with. In general, the plasma is controlled by one or more process parameters, such as: wafer temperature, chamber surface temperature, pressure, reactant gas flow rate, reactant gas mix, reactant gas concentration, gas injection distribution, plasma ion density, and bias voltage. Often such process parameters are optimized for the reactant species on dummy wafers. The optimization of the process parameters and the choice of chemically reactive species are done for the specific process in the form of design of experiments (more often called a DOE).
Although care and caution is involved during the monitoring of a chemical process, more often than not, undesired residues are deposited on the inner surfaces of the process chambers where the processes are taking place. The undesired residues, unfortunately, get deposited in and around the walls of the processing chamber. The build up of residues inside the processing chamber, over time, not only make the processes unreliable and shifted from baseline, but also result in degraded, defective substrates in the form of defects. Without frequent cleaning procedures, impurities from the residue build up can migrate onto the substrate. In addition, process etch rates or deposition rates can vary over time due to the changing chamber conditions from residue build-up resulting in out of control process performance. Thus, the maintenance of the processing equipment is important not only for higher yields and better product performance of current products but also in the development of future generation products.
An important technique to improve the overall quality and efficiency in the processing of devices is to clean the chamber of any deposits. Two methods of cleaning a process chamber are dry etch cleaning and wet cleaning. In the former cleaning operation, process gases are evacuated from the processing chamber and one or more process cleaning gases are introduced. Energy is applied to promote a reaction between the gases and any residues, which may have accumulated on the processing chamber's interior surfaces. Residues on the processing chamber's interior react with the cleaning process gases, forming gaseous by-products which are then exhausted from the processing chamber, along with unreacted portions of the cleaning process gases. The cleaning process is followed by the resumption of normal processing.
An alternative to dry etching i.e., the in-situ cleaning procedure, in which the processing chamber remains sealed, is a wet cleaning procedure that is performed by breaking the processing chamber's vacuum seal and manually wiping down the chamber's interior surfaces. A wet cleaning procedure is normally performed to remove residues that are not entirely removed by the in-situ cleaning process, and tend to accumulate over time. A solvent is sometimes used to dissolve these residues. Once cleaned, the processing chamber is sealed and normal processing is resumed.
Unfortunately, such cleaning operations affect a substrate processing system's utilization in a variety of ways. For example, system utilization is reduced by the time involved in performing cleaning operations. When a wet clean is performed, opening the processing chamber and physically wiping the chamber's interior surfaces results in even more downtime because the processing environment must subsequently be re-stabilized. The re-stabilization of the chamber condition requires processing many wafers to condition the chamber back to the pre-wet clean operating chamber state without excessive residue build-up.
FIG. 1 is a flowchart diagram of the method operations for a composite one step cleaning process for the removal of all chamber deposition byproducts. The method initiates with operation 10 where dummy wafers are processed to check for process readiness. The method then advances to operation 12 where production wafers are processed. Then, the method moves to operation 14 where the etchants for both silicon based byproduct removal and carbon based byproduct removal are combined to run a single step cleaning operation. If there are more wafers to be processed, the production wafers are rerun through operations 12 and 14. The method operations of FIG. 1 can also be performed in wafer-less conditions, in which case it is considered a one-step (composite) wafer-less auto clean (WAC) process. Although in some instances, there may be multiple steps, all these steps are still repeating composite (both silicon based byproduct removal and carbon based byproduct removal together) one-step wafer-less auto clean (WAC) processes.
Although such a recipe has the advantage that the number of process steps can be reduced or not, it fails to ensure complete removal of different byproducts, such as carbon based organic residues and semiconductor based silicon residues. In fact, the removal rates for silicon and carbon deposition are in opposition to each other. The process trends for silicon removal are based primarily on fluorine-based chemistry while carbon removal are based primarily on oxygen-based chemistry. As both fluorine-based and oxygen-based chemistries are mixed together, the removal rates of both silicon and carbon are not at optimal levels. As the O2 concentration increases in the mixture above a certain limit, the silicon removal rate becomes saturated and reduces substantially. Rather both silicon and carbon rates are compromised in a one step composite approach with an immediate removal rate. Thus, the one step approach can not yield optimized removal rates for silicon and carbon deposition. In the past, plasma cleans were used for cleaning etch reactors and deposition reactors with the wafer in the chamber to cover the electrode, but it has become more common to do wafer-less plasma cleans. This has led to the use of a wafer-less auto clean (WAC).
When a composite WAC recipe for both silicon and carbon byproduct removal is used involving a specific mixture of separate etchant gases for silicon removal and for carbon removal, it suffers from non-optimized lower removal rates of both silicon and carbon-based deposition byproducts. That is, the one step composite WAC approach is not tuned to achieve complete by-product removal of both the silicon and the carbon by-products. In effect, the one step composite WAC approach compromises the effectiveness of the removal of the byproducts for the convenience of using a single all-encompassing WAC chemistry. Thus, the composite WAC recipe removes both byproduct materials at a mid-range removal rate that may remove too much of one byproduct and not enough of another byproduct. Consequently, the chamber will be under-cleaned which results in a build up of material, or the chamber will be over-cleaned which results in the degradation of chamber parts and a shortening of their lifetime. Moreover, when fluorine containing cleaning gases are used with the composite WAC recipe, non volatile aluminum fluoride compounds are formed which build up on the inner surfaces of the chamber. The build up of these compounds causes variability of the plasma processing operations performed in the processing chamber. Eventually, the build up can flake off and deposit onto a wafer being processed.
Another shortcoming of frequent cleaning operations is that they tend to increase wear on the processing chamber components when over-cleaning occurs resulting in decreased lifetime of the components. In turn, the cost of consumables increases. Thus, improved methods and apparatus for cleaning semiconductor process chambers are needed. In particular, the cleaning methods and apparatus should be capable of removing the residues created during substrate processing operations, while reducing or eliminating the subsequent formation of cleaning residues.
In view of the foregoing, what is needed is a semiconductor processing chamber configured to optimize chamber byproduct removal of silicon and carbon based byproducts, such that the removal of the silicon based byproducts and the carbon based byproducts is achieved in an efficient manner to provide a consistent environment inside the chamber for each successive semiconductor substrate being processed.